Acoustical absorptive splitter

ABSTRACT

An acoustic absorptive splitter consisting of two absorptive faces separated by a single on-quarter wavelength cavity support is disclosed as an equal substitute for a multi-layered splitter containing a central septum. This construction takes advantage of the reflective properties of the absorptive face sheets to support standing waves in the tuning cavities and in the duct cross modes. The utilization of these design principles permits significant silencer size and weight reduction. It also permits reduced splitter manufacturing costs by elimination of two layers of materials, difficult internal bonding and improvements in quality.

FIELD OF THE INVENTION

[0001] The present invention relates generally to duct silencers and,more particularly, to duct silencers wherein the sound attenuation isachieved through absorption of sound energy by multiple absorptivesurfaces of a longitudinal splitter or group of splitters.

DESCRIPTION OF THE PRIOR ART

[0002] State of Art Absorptive Duct silencers, utilized for lowtemperature applications, such as for commercial air conditioningsystems, engine silencers, etc. utilize fibrous material for lining theduct. Many fiber-like materials may be used, for example, steel wool orglass fiber. The absorptive materials may be confined behind foraminousmaterial such as perforated sheet metal, expanded metal or screening, orsimply glued in place. These silencers, however, are unsuitable for hightemperature gas applications or high velocity gases, since the fiberstend to migrate, fracture or char. They also become blinded or saturatedwhen high amounts of entrained materials such as ash, soot, dust or oilyliquids are present. These materials are particularly unsuitable inaircraft applications, where the danger of fire from oil or fuelcontamination is possible. In addition, it is desirable in theseapplications to minimize weight.

[0003] Duct silencers in aircraft applications utilize metallic sheetabsorber facing materials. These facings are positioned in the duct assplitters and are usually supported by spacing materials. These spacersare designed to create open cavity space behind the facing sheets,honeycomb or “egg-crate” structure being preferred. The facing sheetsmay be adhesively bonded, brazed, welded or mechanically joined to thesesupports. Usually, the support materials are configured to position thefacings a predetermined one-quarter wavelength from a reflective hardwall. The absorber assembly is, thereby, tuned to a desired peak portionof the sound spectrum range; usually one centered on a dominantfrequency emitted by a turbine, fan or other offensive noise generator.It is characteristic of sheet absorbers that they are most efficientwhen the maximum velocity of local standing waves is positioned at theabsorptive surface. The support spacing described above assures that thefacing sheet is properly positioned. It is known that the standing wavevelocity at a reflective wall is zero, and that one-quarter wavelengthfrom this wall it will be a maximum.

[0004] Conventional splitter designs utilize two absorptive faces in aback-to-back arrangement. These splitters are fabricated with twoabsorptive face sheets, a central solid septum, and two one-quarter wavespacer supports. The splitter is then one-half wavelength thick. Thepresence of this thick splitter either forces the duct exterior size tobe increased or creates excessive restriction or blocking, resulting inbackpressure or pressure drop. These size increases also createexcessive increases in weight or force compromises in performance. Inaddition, the solid septum adds weight and requires two internal surfacebonds. This results in significant manufacturing complexity andassociated increased cost.

[0005] An example of an acoustic liner or splitter that utilizes facingmaterials and spacers are described in U.S. Pat. No. 6,209,679, whichissued to Hogeboom, et al. on Apr. 3, 2001 for an “Aircraft engineacoustic liner and method of making same” and U.S. Pat. No. 5,782,082,which issued to Hogeboom, et al. on Jul. 21, 1998 for an “Aircraftengine acoustic liner,” both of which disclose a low resistance acousticliner formed by a middle layer having partitioned cavities sandwichedbetween an imperforate sheet and an optional perforate sheet. A secondarrangement of acoustic linings includes a splitter having a portionformed of low resistance liner. Similarly, U.S. Pat. No. 5,912,442,which issued to Nye, et al. on Jun. 15, 1999 for a “Structure having lowacoustically-induced vibration response” discloses a low vibroacousticstructure comprising first and second facesheet defining a plurality ofholes attached to opposed surfaces of a core defining a plurality ofpassages in communication with the holes to form channels through thestructure.

[0006] Another example of an acoustic liner having a double wall typestructure is described in U.S. Pat. No. 4,294,329, which issued to Rose,et al. on Oct. 13, 1981 for a “Double layer attenuation panel with twolayers of linear type material.” Rose, et al. discloses a double degreeacoustic attenuation sandwich panel having a plurality of stackedcomponents adhered together to form a unitary sandwich structurecomprising an impervious facing of thin sheet material, a firsthoneycomb core with end wise directed cells, a first perforate facing ofthin sheet material, a first thin layer of porous fibrous material, asecond perforated facing of thin sheet material, a second honeycomb corewith end wise directed cells, a third perforate facing of thin sheetmaterial, the second perforated facing sheet having substantially largerperforations than the first and third sheets, and a second layer ofporous fibrous material. A method for manufacturing such a panel isdisclosed in U.S. Pat. No. 4,421,811, which also issued to Rose, et al.on Dec. 20, 1983 for a “Method of manufacturing double layer attenuationpanel with two layers of linear type material.” This patent discloses amethod of manufacturing a double degree acoustic attenuation sandwichpanel having a plurality of stacked components adhered together to forma unitary sandwich structure comprising an impervious facing of thinsheet material, a first honeycomb core with end wise directed cells, afirst perforate facing of thin sheet material, a first thin layer ofporous fibrous material, a second perforated facing of thin sheetmaterial, a second honeycomb core with end wise directed cells, a thirdperforate facing of thin sheet material, the second perforated facingsheet having substantially larger perforations than the first and thirdsheets, and a second layer of porous fibrous material.

[0007] Sound absorbing panels are described in both U.S. Pat. No.4,667,768, which issued to Wirt on May 26, 1987 for a “Sound absorbingpanel” and U.S. Pat. No. 4,084,366, which issued to Saylor, et al. onApr. 18, 1978 for a “Sound absorbing panel.” The former discloses asound absorbing panel comprising an array of walls configured to providetwo or more contiguous hollow cells having adjacent open ends andadjacent closed impermeable ends, the open ends defining a soundreceiving end of the array, at least one impermeable three dimensionalclosed surface disposed in at least one of the cells, and a flowresistive permeable facing sheet covering the sound receiving end. TheSaylor et al. patent, on the other hand, discloses a wall panel having arigid rectangular frame with a core structure disposed within the regionbounded by the frame, which core structure comprises at least onehoneycomb layer and sheetlike skins fixedly secured to opposite sides ofthe frame and extending across the region bounded by the frame forconfining the honeycomb layer therebetween.

[0008] The particular applicability of such acoustic liners for jetengines is shown in U.S. Pat. No. 5,594,216, which issued to Yasukawa,et al. on Jan. 14, 1997 for a “Jet engine sound-insulation structure,”and which discloses a jet engine sound insulator having a structure thatincludes a framework and an acoustical insulation material disposedwithin the framework composed of a rigid matrix of randomly oriented,fused silica fibers having fiber diameters predominantly in the range2-8 .mu.m, a three-dimensionally continuous network of open,intercommunicating voids, a flow resistivity between about 10-200Krayls/m, and a density of between about 2 and 8 lb/ft3.

[0009] Such devices, however, fail to offer the unique advantages of theacoustical absorptive splitter as contemplated by the present invention,namely, to provide an effective absorptive splitter that is thinner incross section, easier to manufacture and available at reduced costs.

[0010] The following publications are all directed to the scientific andengineering elements of acoustics, and are incorporated herein byreference: (1) “Notes on Sound Absorption Technology”, K. U. Ingard.Noise Control Foundation, Poughkeepsie, N.Y., 1994 ISBN 0-931784-28-X;(2) “Elements of Acoustical Engineering” Harry F. Olson, D. Van NostrandCompany, New York, N.Y. 1947; (3) “Acoustical Engineering” Harry F.Olson, D. Van Nostrand Company, Inc., New York, 1957, Library ofCongress Card No. 57-8143; and (4) “Handbook of Noise Control” Cyril M.Harris, McGraw-Hill Book Company, New York, N.Y. 1957 Library ofCongress Catalog Card No. 56-12268

SUMMARY OF THE INVENTION

[0011] The present invention provides an absorptive splitter thatconsists of two absorptive face sheets spaced apart one-quarterwavelength by a single honeycomb or egg-crated supporting structure. Theinvention eliminates the solid septum and its two bonded joints. It alsoeliminates one of the quarter-wave thick supports required in state ofart splitters. The new splitter is then substantially (almost 50%)thinner and provides the advantage of reduced weight and simplifiedmanufacturing. It also provides reduced overall duct size, pressure dropand back pressure.

[0012] Noise attenuation testing in laboratory ducts has shown that theattenuation performance of this design is equal to or better than thestate of art design. This result contradicts present theory in that theone-quarter wavelength cavity between the two face sheets is active andcontinues to control tuning; this despite the absence of a central hardwall reflector. This unexpected result indicates that the two absorptiveface sheets are sufficiently reflective to maintain the internalstanding wave.

[0013] These and other aspects of the invention will become apparentfrom the following detailed description and the accompanying drawings,which illustrate by way of example the features of the invention and itsapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a plot of the acoustic impedance of Fibermetal measuredwith an acoustic impedance tube instrument;

[0015]FIG. 2 is a drawing showing the structure of a typical state ofart acoustical splitter as used in a duct silencer;

[0016]FIG. 3 is a drawing showing a duct containing an acoustic splitteras described in FIG. 2;

[0017]FIG. 4 is a drawing showing the simplified structure that replacesthe splitter shown in FIG. 2 and is the subject of this patent;

[0018]FIG. 5 is a drawing showing a duct containing an acoustic splitterof the type shown in FIG. 4;

[0019]FIG. 6 is an end view of an acoustical silencer duct containing astate of art splitter of the type shown in FIG. 2;

[0020]FIG. 7 is an end view of an acoustical silencer duct containing asimplified splitter as shown in FIG. 4;

[0021]FIG. 8 is an end view of an acoustical silencer duct containingmultiple splitters of the type shown in FIG. 2;

[0022]FIG. 9 is an end view of an acoustical silencer duct containingmultiple splitters of the type shown in FIG. 4;

[0023]FIG. 10 is an end view of an acoustical silencer duct containing asplitter of the type shown in FIG. 2 plus treated side walls;

[0024]FIG. 11 is an end view of an acoustical silencer duct containing asplitter of the type shown in FIG. 4 plus treated side walls;

[0025]FIG. 12 is an end view of a circular acoustical silencer ductcontaining a splitter of the type shown in FIG. 4; and

[0026]FIG. 13 is an end view of a hexagonal acoustical silencer ductcontaining a splitter of the type shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Duct silencers for use in aircraft have been designed andmanufactured generally as shown and configured in FIG. 6, FIG. 8 andFIG. 10. These may contain acoustical absorptive splitters in proximityto the flowing gases as shown in these figures. The splitters are tunedby sizing the spacing established by the thickness of the cavities 40 insupporting structure behind the face sheets 50. This spacing is referredto in the industry as the “cavity depth” 42. The cavity depth 42 isdesigned to approximate one-quarter wavelength at a desired peakfrequency. This spacing establishes a peak resonant standing wavefrequency in the cavity 40 and, thereby, a peak absorptive frequency forthe assembly. Usually this frequency is chosen to be the first harmonicof the dominant noise being treated. For example, it could be the bladepassing frequency of a fan or turbine, or it could be a center frequencyof a noise spectrum of a hiss from an air blast.

[0028] The tuning mechanism involved in the cavity depth design is basedon the establishment of a standing wave in the cavity 40 behind the facesheets 50. Some of the energy from the sound wave that impinges on theface sheets 50 penetrates through the face sheets 50 and excites andperpetuates a standing wave in the cavity 40. The state of arttermination of the cavity 40, the backing plate or septum 30, reflectsenergy and its face becomes a zero velocity, maximum pressure point. Acorresponding point, one-quarter wavelength away, will then be at themaximum velocity of the standing wave. The media face sheet 50 islocated at this point. Further, as a consequence, the face sheets 50 isthen at a maximum velocity point and the gas is forced to cyclicallypermeate through the porous face sheet. The tortuous resistive paththrough the face sheets 50 converts the sound energy to heat and therebyabsorbs the sound.

[0029] The face sheet 50 is designed to offer an acoustical impedancematch R to that of the gas, ρc, in the duct 20. When that match iscorrect, as measured in an impedance tube or by air permeability, theorysays that the face sheets 50 will not reflect sound, but will optimallyabsorb it. Ingard states, pages 1-3, that if the absorption coefficient,α, of a material is measured, it's value will be α=1−(R)², where R isthe reflection coefficient. If α=1, then R must be zero or very small.

[0030]FIG. 1 is a plot of the normalized resistance and reactance of atypical absorber face sheet 50 versus frequency as measured in anacoustical impedance tube. The resistance curve is equal to 1.0, theoptimum match, i.e., the acoustical resistance of the face sheet 50matches the acoustical impedance of air. This match is displayed for asubstantial useable range of the test frequencies. The impedance plotfor this material then indicates that reflectance must be very low. Thereactance curve indicates the tuned response of the sample and theresonant frequency of the supporting cavity 40. This occurs at thefrequency (2750 Hertz) where the reactance curve crosses zero. This facesheet 50 material should then be non-reflective and, if used toterminate the tuning cavity 40, should not support a standing wave orcontrol tuning.

[0031] However, contrary to this theory, the splitter 10 of the presentinvention, as illustrated in FIG. 4, provides a tuning curve similar totests of the “state of art” splitter as shown in FIG. 2. The splitter 10of the present invention eliminates the backing plate 30 and insteadutilizes a pair of acoustically matched absorptive face sheets 52separated by supporting cavities 40. The acoustically matched materialin the face sheets 50 of the present invention is sufficientlyreflective to support the tuning standing wave.

[0032] In the preferred embodiment, the absorptive face sheets 52 shouldbe thin foraminous sheet materials, and may be fabricated out of avariety of materials, although test have shown that either perforatedmetal, fibermetal, metal screening or a combination of these absorptivemetals is preferred. For high temperature applications or in aircraft,the absorptive face sheets 52 may be constituted of fibrous metalsconsisting of randomly oriented or felted fibers, closely woven metalscreens, perforated metals or layers of any or all of these. Thesematerials may be metallurgically sinter or diffusion bonded or bondedwith resins. In some applications, mechanical bonds or simple layeringmay be adequate. Low temperature applications may use resin bonded openweave fiberglass or carbon fiber. In general, however, any thin, porous,sheet material or sandwich that is impedance matched to ½ to 2 ρc of theambient air and is sufficiently rigid to provide internal cavityreflection will suffice. It should be appreciated that while anon-metallic material may be utilized for the absorptive face sheets 52,for example in those situations where weight considerations play apivotal role, such non-metallic material must display the sameabsorptive and reflective qualities of the metallic absorptive facesheets 52 of the preferred embodiment. Soft cloth, felt or silk-likematerial is not recommended.

[0033] The cavities 40 are supported by a cavity support structure 44arranged in a variety of configurations. In the preferred embodiment,the cavity support structures 44 are configured as a honeycomb corematerial, or an “egg-crate”-type structure. Generally, any cell-likestructure that will adequately support the absorptive face sheets 52 andprovide wall structures to guide the acoustical standing waves into anormal approach to the absorptive face sheets 52 will be satisfactory.The “cell size” in the preferred embodiment is on the order of one-halfthe cavity depth 42. The walls may be metal or non-metallic.

[0034] The absorptive face sheets 52 may be joined by a variety ofmethods, including welding, brazing, resin bonding or mechanicalattachment by crimping or through the use of bolts, rivets or screws.Regardless of the method, however, care should be taken to minimize thebinding of the absorptive face sheets 52 from excessive wicking of brazealloys, adhesive resins or mechanical attachment means.

[0035] The fact that the acoustically matched material in the absorptiveface sheets 52 of the present invention is sufficiently reflective tosupport the tuning standing wave, makes the acoustical absorptivesplitter 10 of the present invention ideal for use as a silencer. First,the assembly without the septum 30, as shown in FIG. 4, is much simplerand easier to manufacture, since the solid central septum 30, two brazeor adhesive bond joints 32 and one of the cavity supports (not shown)have been eliminated. The thinner splitter 10 presents less restrictionto flow and, therefore, reduces pressure drop. In addition to the easeof manufacture, the elimination of the septum 30, joints 32 and cavitysupports significantly reduces the weight of the splitter 10 of thepresent invention, making it ideal for aircraft applications.Manufacturing cost is likewise reduced through both elimination ofunneeded materials and reduction in complexity. The design of thesplitter 10 of the present invention also facilitates quality control,since the splitter 10 does not contain the two hidden central bondjoints 32. Finally, the splitter 10 assembly can be of all weldedconstruction utilizing state of art processing, which is not possiblewith the old design.

[0036] By illustration of the simplification permitted through the useof the splitter 10 of this invention, FIG. 6 and FIG. 7 show comparativeend views of single splitter duct silencers, FIG. 6 illustrating thecurrent “state of the art” splitter using a central backing plate 30 andFIG. 7 illustrating the splitter 10 of the present invention. In bothfigures, the duct cross sections 20 have been sized as one-quarterwavelength, thereby placing maximum pressure at the exterior walls 70and maximum velocity at the faces 50, 52 of the splitter. FIG. 7 showsthe significant size reduction achieved by the splitter 10 of thepresent invention.

[0037]FIG. 8 and FIG. 9 illustrate the use of multiple splitters in thesame duct 20, FIG. 8 illustrating the “state of the art” splitter andFIG. 9 illustrating the splitter 10 of the present invention. Referringto FIG. 8, the sizes of the ducts 20, at the sidewalls, are one-quarterwavelength and the central duct 60, is one-half wavelength. The centralduct 60 is made wider in the “state of the art” splitter based on theassumption that the faces 50 are non-reflective. This spacingestablishes a full wavelength between the two solid septa 30 and placesmaximum velocity points at both absorptive faces 50.

[0038] While the spacing illustrated in FIG. 8 can certainly be utilizedwith the new design splitter 10 of the present invention, if thepressure drop constraints will allow it, the arrangement shown in FIG. 9can also be used. In this configuration, the absorptive faces 52 aresufficiently reflective to support quarter wave standing waves acrossthe duct 20, as was found with the cavities 40. The permits significantsize and weight reduction.

[0039]FIG. 10 and FIG. 11 illustrate yet another configuration, FIG. 10illustrating the “state of the art” splitter and FIG. 11 illustratingthe splitter 10 of the present invention. Referring to FIG. 10, thecentral duct 60 sizes are half wavelength, and quarter wave treatmentsare applied to the sidewalls. Again, while this same spacing can also beutilized by the new design splitter 10 of the present invention, FIG. 11shows an alternate configuration, where the reflective properties of theabsorptive face sheets 52 have been utilized.

[0040] The acoustical absorptive splitter 10 of the resent invention ispreferably used in applications where the noise frequencies are above450 Hertz. This includes applications such as gas turbine inlet andexhaust, fan noise (particularly when blade tip velocities exceed Machone), hiss from the blow-down of high pressure gases, and high velocityor high temperature duct applications. The splitters 10 performparticularly well in non-rectangular ducts, i.e., round, oval,hexagonal, etc., as is illustrated in FIGS. 12 and 13. The diffusednature of the reflected waves from the curved or angled walls of theduct provide excellent excitation of the standing waves in the tunedsplitter cavities 40. Tests have shown that the splitter 10 also workswell with non-symmetrical walls. It is therefore not necessary to havenormally oriented standing waves reflected from flat wall surfaces toachieve high insertion loss. The main criteria is the ratio of treatedsurface area to duct cross-sectional area.

[0041] Furthermore, crossed splitters 10 that are tuned to complementarywavelengths are particularly useful. The cavity depth 42 of a splitter10 will provide maximum insertion loss at the frequency approximated bythe quarter wave tuning. The splitter 10 will provide very low insertionloss at the frequency corresponding to the corresponding half wavetuning, i.e., absorption peaks will occur at the first, third, fifth,seventh, etc., frequencies with deep gaps between. These gaps can befilled by the use of a second splitter that is one-half as thick as thefirst. It will tune to these gaps and provide insertion loss at thesecond, fourth, sixth, etc. frequencies. The result is a broadbandsilencer that is simple and inexpensive.

[0042] Having thus described the invention with particular reference tothe preferred forms thereof, it will be obvious that various changes andmodifications can be made therein without departing from the spirit andscope of the present invention as defined by the appended claims.

What is claimed is:
 1. An acoustic splitter for use in attenuating soundthrough the absorption of sound energy having a fixed wavelength, saidsplitter comprising a plurality of one-quarter wavelength cavitiesdefined by a cavity support structure, said cavities being bounded oneither end by an absorptive face sheet.
 2. The acoustic splitter ofclaim 1, wherein said absorptive face sheets are composed of perforatedmetal.
 3. The acoustic splitter of claim 1, wherein said absorptive facesheets are composed of fibermetal.
 4. The acoustic splitter of claim 1,wherein said absorptive face sheets are composed of metal screening. 5.The acoustic absorptive splitter of claim 1, wherein said absorptiveface sheets are composed of a combination of absorptive materials. 6.The acoustic splitter of claim 1, wherein said absorptive face sheetsare composed of a porous non-metallic material.
 7. The acoustic splitterof claim 1, wherein said cavity support is arranged in a honeycombconfiguration.
 8. The acoustic splitter of claim 1, wherein said cavitysupport is arranged in an egg crate-type configuration.
 9. An acousticalsilencer for use in attenuating sound through the absorption of soundenergy having a fixed wavelength, said acoustical silencer comprising atleast two acoustic splitters comprising a plurality of one-quarterwavelength cavities defined by a cavity support structure, said cavitiesbeing bounded on either end by an absorptive face sheet, wherein saidsplitters are spaced at quarter wavelengths.
 10. An acoustical silencerfor use in attenuating sound through the absorption of sound energyhaving a fixed wavelength, said acoustical silencer comprising first andsecond acoustic splitters comprising a plurality of one-quarterwavelength cavities defined by a cavity support structure, said cavitiesbeing bounded on either end by an absorptive face sheet, wherein saidsplitters are spaced at quarter wavelengths, said second acousticsplitter having a thickness one half the thickness of said firstacoustic splitter.